1. Technical Field
The present invention relates to a technique which applies a drive signal to drive a capacitive load such as a piezoelectric element.
2. Related Art
Most of piezoelectric elements are driven by a predetermined drive signal applied thereto, as in an ejection head which is mounted to an ink jet printer, or the like. In general, such a piezoelectric element is driven by a drive signal generated by power-amplifying a drive waveform signal.
As a method of generating the drive signal by power-amplifying the drive waveform signal, a method of using a digital power amplifier has been proposed which is different from a method of using an analog power amplifier. The method of using the digital power amplifier can perform the power amplification with small power loss and can be easily made smaller, compared with the method of using an analog power amplifier (for example, JP-A-2005-329710). In order to generate the drive signal using the digital power amplifier, firstly, the drive wavelength signal is pulse-modulated to be converted into a modulated signal of a pulse waveform. Then, the obtained modulated signal is power-amplified by the digital power amplifier to be converted an amplified digital signal. Thereafter, modulated components included in the amplified digital signal are removed by a low pass filter, to thereby generate the power-amplified drive signal.
Further, most of drive signals include a period in which a voltage value is not changed. Since the piezoelectric element to which the drive signal is applied is a capacitive load, it is not necessary to supply electric power in the period in which the voltage value of the drive signal is not changed. In other words, operation of the digital power amplifier in this period becomes wasteful. In this regard, a technique has been proposed in which the operation of the digital power amplifier is stopped in the period in which the voltage value of the drive signal is not changed to further reduce power loss in generation of the drive signal (JP-A-2011-005733).
However, the previously proposed technique has a problem that the voltage value of the drive signal may be changed when the operation of the digital power amplifier is stopped. If the voltage value is changed in the period in which the voltage value of the drive signal should be constant, it is difficult to drive the capacitive load such as a piezoelectric element with high accuracy.